Memory cell having magnetic tunnel junction and thermal stability enhancement layer

ABSTRACT

A magnetoresistive random-access memory (MRAM) device is disclosed. The device described herein has a thermal stability enhancement layer over the free layer of a magnetic tunnel junction. The thermal stability enhancement layer improves the thermal stability of the free layer, increases the magnetic moment of the free layer, while also not causing the magnetic direction of the free layer to become in plan. The thermal stability enhancement layer can be comprised of a layer of CoFeB ferromagnetic material.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of Provisional Application No.62/287,994, filed Jan. 28, 2016. Priority to this provisionalapplication is expressly claimed, and the disclosure of the provisionalapplication is hereby incorporated herein by reference in its entirety.

FIELD

The present patent document relates generally to spin-transfer torquemagnetic memory (STT-MRAM) devices and, more particularly, to STT-MRAMdevices having thermal stability enhancement layer that increases thethermal stability of the free layer of a magnetic tunnel junction.

BACKGROUND

Magnetoresistive random-access memory (“MRAM”) is a non-volatile memorytechnology that stores data through magnetic storage elements. In a typeof MRAM, the magnetic storage elements comprise two ferromagnetic platesor electrodes that can hold a magnetic field and are separated by anon-magnetic material, such as a non-magnetic metal or insulator. Such astructure is called a magnetic tunnel junction (“MTJ”). In general, oneof the plates has its magnetization pinned (i.e., a “reference layer”),meaning that this layer has a higher coercivity than the other layer andrequires a larger magnetic field or spin-polarized current to change theorientation of its magnetization. The second plate is typically referredto as the free layer and its magnetization direction can be changed by asmaller magnetic field or spin-polarized current relative to thereference layer. Thus, the free layer is also referred to as the storagelayer. MTJ's are manufactured using stacked materials, with each stackof materials forming an MTJ pillar.

MRAM devices store information by changing the orientation of themagnetization of the free layer. In particular, based on whether thefree layer is in a parallel or anti-parallel alignment relative to thereference layer, either a “1” or a “0” can be stored in each MRAM cell.Due to the spin-polarized tunneling magnetoresistance (TMR) effect, theelectrical resistance of the cell change due to the orientation of themagnetic fields of the two layers. The cell's resistance will bedifferent for the parallel and anti-parallel states and thus the cell'sresistance can be used to distinguish between a “1” and a “0”. Oneimportant feature of MRAM devices is that they are non-volatile memorydevices, since they maintain the information even when the power is off.The two plates can be sub-micron in lateral size and the magnetizationdirection can still be stable with respect to thermal fluctuations.

Spin transfer torque or spin transfer switching, uses spin-aligned(“polarized”) electrons to change the magnetization orientation of thefree layer in the magnetic tunnel junction. In general, electronspossess a spin, a quantized number of angular momentum intrinsic to theelectron. An electrical current is generally unpolarized, i.e., itconsists of 50% spin up and 50% spin down electrons. Passing a currentthough a magnetic layer polarizes electrons with the spin orientationcorresponding to the magnetization direction of the magnetic layer(i.e., polarizer), thus produces a spin-polarized current. If aspin-polarized current is passed to the magnetic region of a free layerin the magnetic tunnel junction device, the electrons will transfer aportion of their spin-angular momentum to the magnetization layer toproduce a torque on the magnetization of the free layer. Thus, this spintransfer torque can switch the magnetization of the free layer, which,in effect, writes either a “1” or a “0” based on whether the free layeris in the parallel or anti-parallel states relative to the referencelayer.

MRAM devices are considered as the next generation structures for widerange of memory applications. One MRAM technology uses a perpendicularmagnetic tunnel junction device. In perpendicular MTJ devices, the freeand reference layers are separated by a thin insulator layer for spinpolarized tunneling. The free and reference layers have a magneticdirection that is perpendicular to their planes, thus creating aperpendicular magnetic tunnel junction (pMTJ). The pMTJ configurationmay provide a lower critical switching current when compared to in-planeMTJ technology, simplified layer stack structure without need of usingthick antiferromagnetic layers, and reduction of the device size below40 nm.

FIG. 1 illustrates a pMTJ stack 100 for a conventional MRAM device. Asshown, stack 100 includes one or more seed layers 110 provided at thebottom of stack 100 to initiate a desired crystalline growth in theabove-deposited layers. A perpendicular synthetic antiferromagneticlayer (“pSAF layer”) 120 is disposed on top of the seed layers 110. MTJ130 is deposited on top of synthetic antiferromagnetic (SAF) layer 120.MTJ 130 includes reference layer 132, which is a magnetic layer, anon-magnetic tunneling barrier layer (i.e., the insulator) 134, and thefree layer 136, which is also a magnetic layer. It should be understoodthat reference layer 132 is actually part of SAF layer 120, but formsone of the ferromagnetic plates of MTJ 130 when the non-magnetictunneling barrier layer 134 and free layer 136 are formed on referencelayer 132. As shown in FIG. 1, magnetic reference layer 132 has amagnetization direction perpendicular to its plane. As also seen in FIG.1, free layer 136 also has a magnetization direction perpendicular toits plane, but its direction can vary by 180 degrees.

The first magnetic layer 114 in the perpendicular SAF layer 120 isdisposed over seed layer 110. Perpendicular SAF layer 120 also has anantiferromagnetic coupling layer 116 disposed over the first magneticlayer 114. As seen by the arrows in magnetic layers 114 and 132 ofperpendicular SAF 120, layers 114 and 132 have a magnetic direction thatis perpendicular to their respective planes. Furthermore, a nonmagneticspacer 140 is disposed on top of MTJ 130 and a polarizer 150 mayoptionally be disposed on top of the nonmagnetic spacer 140. Polarizer150 is a magnetic layer that has a magnetic direction in its plane, butis orthogonal to the magnetic direction of the reference layer 132 andfree layer 136. Polarizer 150 is provided to polarize a current ofelectrons (“spin-aligned electrons”) applied to pMTJ structure 100.Further, one or more capping layers 160 can be provided on top ofpolarizer 150 to protect the layers below on MTJ stack 100. Finally, ahard mask 170 is deposited over capping layers 160 and is provided topattern the underlying layers of the MTJ structure 100, using a reactiveion etch (RIE) process.

As discussed, one type of MTJ is referred to as a perpendicular MTJ. Ina perpendicular MTJ, the reference layer and the free layer each have amagnetic direction that is perpendicular to the plane of theirrespective layers. The resistance of the magnetic memory device issensitive to the relative orientation of the magnetization vector of thefree magnetic layer and the magnetization vector of the reference layer.The resistance of the magnetic memory device is highest when themagnetization vectors of the free magnetic layer and the referencelayer, respectively, are in anti-parallel alignment. The resistance ofthe magnetic device is lowest when the magnetization vectors of thelayers free magnetic layer and the reference layer, respectively, are inparallel alignment. Thus, a resistance measurement or its equivalent candetermine the orientation of the magnetization vector of the freemagnetic layer.

An important characteristic of MTJs is thermal stability. The thermalstability of each perpendicular MTJ, i.e., the magnetic bits, isproportional to the magnetic material volume of the MTJ for a givenperpendicular anisotropy. Thermal stability of an MTJ is a factor indata retention capability. Thus, improving the thermal stability of thefree layer of an MTJ is an important design consideration. Because ofthe relationship between the magnetic material volume of an MTJ and theperpendicular anisotropy, as MTJ pillar dimensions decreases, forexample when shrinking an existing design for future generation MRAMdevices, the thermal stability declines. This is highly undesirable.Unfortunately the thickness of the free layer cannot be increased atwill to add more magnetic moment (volume) to enhance the thermalstability. Thus, the thermal stability of the free layer structure witha perpendicular magnetic direction cannot be enhanced simply byincreasing the thickness of the material used to construct the freelayer (typically CoFeB). This is because there is a limit on thethickness of CoFeB where the perpendicular anisotropy can be obtained.For CoFeB, this thickness may be around sixteen (16) Angstroms. Abovethis thickness, the magnetization reverses to be in plane, meaning thatthe MTJ will no longer be a perpendicular MTJ. Thus, the thermalstability of the perpendicular MTJ free (i.e., storage) layer cannot beenhanced by further increasing the free layer thickness.

Perpendicular magnetization direction can be achieved using surfaceperpendicular anisotropy (interface perpendicular magnetic anisotropy)which is an interface property of the ferromagnetic film and neighboringcapping and seeding layer of non-magnetic material used for a freelayer. Interface perpendicular magnetic anisotropy (IPMA) is inverselyproportional to the thickness of the film. For common ferromagneticmaterials, IPMA becomes strong enough to keep magnetization out of planein the thickness range of 1.2 to 1.6 nm. However, at this thicknessesrange, the magnetic moment of the free layer is small. This smallmagnetic moment of the free layer reduces thermal stability. On theother hand, increasing the free layer thickness lowers the IPMA, whichcauses the free layer to become in-plane magnetized. In a perpendicularMTJ device, this is not acceptable since it would cause degradation ofthe tunneling magnetoresistance (TMR) value to a level below whichdevice can operate reliably. Thus, increasing the free layer thicknesslowers the thermal stability by diminishing the perpendicularanisotropy. In addition, the device itself becomes useless, as the freelayer loses its perpendicular magnetic anisotropy. This is one of themost difficult issues to address for perpendicular MTJ MRAM devices.

Thus, a need exists to enhance the thermal stability of the free layerof an MTJ where the thickness of the free layer does not have to bedisturbed.

SUMMARY

An MRAM device is disclosed that comprises a thin layer of magneticmaterial having perpendicular anisotropy, referred to herein as athermal stability enhancement (TSE) layer, deposited on a non-magneticseparation layer, where the non-magnetic separation layer is locatedbetween the free layer and the TSE layer. The TSE layer magnetizationcan be optimized as described herein to enhance the switchingcharacteristics of the free layer.

In an embodiment, a magnetic device is disclosed. The embodimentcomprises a bottom electrode in a first plane. The embodiment furthercomprises a perpendicular synthetic antiferromagnetic structureincluding a magnetic reference layer in a second plane, where themagnetic reference layer has a magnetization direction that isperpendicular to the second plane and having a fixed magnetizationdirection. The embodiment further discloses a non-magnetic tunnelbarrier layer in a third plane, which is disposed over the magneticreference layer. The embodiment further comprises a a free magneticlayer in a fourth plane that is disposed over the non-magnetic tunnelbarrier layer. The free magnetic layer has a magnetization vector thatis perpendicular to the fourth plane and has a magnetization directionthat can switch from a first magnetization direction to a secondmagnetization direction. The magnetic reference layer, the non-magnetictunnel barrier layer and the free magnetic layer forming a magnetictunnel junction. The embodiment further comprises a non-magnetic thermalstability enhancement coupling layer in a fifth plane that is disposedover the free magnetic layer. The embodiment also comprises a magneticthermal stability enhancement layer in a sixth plane that is physicallyseparated from the free magnetic layer and coupled to the free magneticlayer by the non-magnetic thermal stability enhancement coupling layer.The magnetic thermal stability enhancement layer has a magnetizationdirection that is perpendicular to the sixth plane and has amagnetization direction that can switch from the first magnetizationdirection to the second magnetization direction, wherein switching ofthe magnetic thermal stability enhancement layer from the firstmagnetization direction to the second magnetization direction tracksswitching in the magnetic free layer. The embodiment also comprises acap layer in a seventh plane that is disposed over the thermal stabilityenhancement layer.

In an aspect of the embodiment, the magnetic device further comprises acurrent source that directs electrical current through the cap layer inthe seventh plane, the magnetic thermal stability enhancement layer in asixth plane, the non-magnetic thermal stability enhancement couplinglayer in the fifth plane, the free magnetic layer in the fourth plane,the non-magnetic tunnel barrier layer in the third plane, the magneticreference layer in the second plane, and the bottom electrode in thefirst plane.

In another aspect of the embodiment, the magnetic thermal stabilityenhancement layer comprises a layer of CoFeB.

In another aspect of the embodiment, the magnetic thermal stabilityenhancement layer comprises a film of CoFeB having a thickness between1.3 nanometers and 1.5 nanometers.

In another aspect of the embodiment, the free magnetic layer comprisesCoFeB with a Ta interlayer.

In another aspect of the embodiment, the free magnetic layer has a sumthickness of 1.6 nanometers.

In another aspect of the embodiment, the perpendicular syntheticantiferromagnetic structure further comprises a first magnetic pSAFlayer and a second magnetic pSAF layer, where the first magnetic pSAFlayer is over the first electrode and is separated from the secondmagnetic pSAF layer by an exchange coupling layer.

In another aspect of the embodiment, a ferromagnetic coupling layer isin between the second magnetic pSAF layer and the magnetic referencelayer.

In another aspect of the embodiment, the magnetic thermal stabilityenhancement layer is magnetically coupled to the free magnetic layer bythe non-magnetic thermal stability enhancement coupling layer.

In another aspect of the embodiment, the non-magnetic thermal stabilityenhancement coupling layer comprises a layer MgO.

In another aspect of the embodiment, the layer of MgO has a thicknessbetween 0.6-1.2 nm.

In another aspect of the embodiment, the layer of MgO has a thickness0.7 nm.

In another aspect of the embodiment, the non-magnetic thermal stabilityenhancement coupling layer provides high interface perpendicularmagnetic anisotropy between the magnetic thermal stability enhancementlayer and free magnetic layer such that the magnetic direction of thefree magnetic layer remains perpendicular to the fourth plane and themagnetic direction of the magnetic thermal stability enhancement layerremains perpendicular to the sixth plane.

In another embodiment, a magnetic device is disclosed that comprises aperpendicular magnetic tunnel junction having a magnetic reference layerand a magnetic free layer. The magnetic reference layer and the magneticfree layer are separated by a non-magnetic tunneling barrier layer. Themagnetic reference layer has a fixed magnetic direction that isperpendicular to its plane. The magnetic free layer has a variablemagnetic direction that can switch between a first perpendicularmagnetic direction and second perpendicular magnetic direction. Thefirst perpendicular magnetic direction and the second perpendicularmagnetic direction is perpendicular to the magnetic free layer. Theembodiment further comprises a magnetic thermal stability enhancementlayer disposed over the magnetic free layer of the magnetic tunneljunction. The magnetic thermal stability enhancement layer comprises amagnetic material having a variable magnetic direction that can switchbetween the first perpendicular magnetic direction and the secondperpendicular magnetic direction. In an embodiment, the switching of themagnetic thermal stability enhancement layer from the firstmagnetization direction to the second magnetization direction tracksswitching in the magnetic free layer. The embodiment further comprises anon-magnetic thermal stability enhancement coupling layer disposed inbetween and physically separating the magnetic free layer of themagnetic tunnel junction and the magnetic thermal stability enhancementlayer. The non-magnetic thermal stability enhancement coupling layermagnetically couples the free magnetic layer and the magnetic thermalstability coupling layer.

In aspect of the embodiment, the magnetic device further comprises anelectrode and a perpendicular synthetic antiferromagnetic structurecoupled to the electrode. The perpendicular synthetic antiferromagneticstructure includes the magnetic reference layer. In an aspect of thisembodiment, a cap layer is disposed over the magnetic thermal stabilityenhancement layer.

In another aspect of the embodiment, the perpendicular syntheticantiferromagnetic structure further comprises a first magnetic pSAFlayer and a second magnetic pSAF layer. The first magnetic pSAF layer isover the electrode and is separated from the second magnetic pSAF layerby a non-magnetic exchange coupling layer.

In another aspect of the embodiment, the magnetic thermal stabilityenhancement layer comprises CoFeB.

In another aspect of the embodiment, the magnetic thermal stabilityenhancement layer comprises a film of CoFeB having a thickness between1.3 nanometers and 1.5 nanometers.

In another aspect of the embodiment, the free magnetic layer comprisesCoFeB with a Ta interlayer.

In another aspect of the embodiment, the free magnetic layer has a sumthickness of 1.6 nanometers.

In another aspect of the embodiment, the magnetic device furthercomprises a current source that directs electrical current through thecap layer, the magnetic thermal stability enhancement layer, thenon-magnetic thermal stability enhancement coupling layer, the freemagnetic layer, the non-magnetic tunnel barrier layer, the perpendicularsynthetic antiferromagnetic structure, and the electrode.

In another aspect of the embodiment, the non-magnetic thermal stabilityenhancement coupling layer provides high interface perpendicularmagnetic anisotropy between the magnetic thermal stability enhancementlayer and free magnetic layer such that the magnetic direction of thefree magnetic layer and the magnetic direction of the magnetic thermalstability enhancement layer remain out-of-plane.

These and other objects, features, aspects, and advantages of theembodiments will become better understood with reference to thefollowing description and accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included as part of the presentspecification, illustrate the presently preferred embodiments and,together with the general description given above and the detaileddescription given below, serve to explain and teach the principles ofthe MTJ devices described herein.

FIG. 1 illustrates a perpendicular MTJ stack for an MRAM device withorthogonal polarizing layer.

FIG. 2 illustrates a process for manufacturing a magnetic device usingthe concepts described herein.

FIG. 3 illustrates the various layers of a magnetic device manufacturedin accordance with the teachings described herein.

FIG. 4 is a graph of the thin film vibrating sample magnetometer (VSM)major hysteresis loop data for various perpendicular magnetic tunneljunction devices.

FIG. 5 is a graph comparing the thermal stability of devices that have athermal stability enhancement layer against devices without a thermalstability enhancement layer.

The figures are not necessarily drawn to scale and the elements ofsimilar structures or functions are generally represented by likereference numerals for illustrative purposes throughout the figures. Thefigures are only intended to facilitate the description of the variousembodiments described herein; the figures do not describe every aspectof the teachings disclosed herein and do not limit the scope of theclaims.

DETAILED DESCRIPTION

The following description is presented to enable any person skilled inthe art to create and use an STT-MRAM device using a perpendicularmagnetic tunnel junction having a free layer with high thermalstability. Each of the features and teachings disclosed herein can beutilized separately or in conjunction with other features to implementthe disclosed system and method. Representative examples utilizing manyof these additional features and teachings, both separately and incombination, are described in further detail with reference to theattached drawings. This detailed description is merely intended to teacha person of skill in the art further details for practicing preferredaspects of the present teachings and is not intended to limit the scopeof the claims. Therefore, combinations of features disclosed in thefollowing detailed description may not be necessary to practice theteachings in the broadest sense, and are instead taught merely todescribe particularly representative examples of the present teachings.

In the following description, for purposes of explanation only, specificnomenclature is set forth to provide a thorough understanding of thepresent teachings. However, it will be apparent to one skilled in theart that these specific details are not required to practice the presentteachings.

An embodiment of an STT-MRAM device using the present teachings willdescribed with reference to FIGS. 2 and 3. FIG. 2 is a flow chartshowing a method 200 for manufacture of an STT-MRAM device 300 using thepresent teachings. FIG. 3 illustrates the various layers of a STT-MRAMdevice 300 manufactured in accordance with the teachings describedherein. It is noted that FIG. 3 is illustrated with space between eachof the layers. A person having skill in the art will recognize that thegaps are for illustration purposes only, and that an actual STT-MRAMdevice will not have gaps between its various layers. Note that thevarious layers illustrated in FIG. 3 are for an exemplary device. Aperson having ordinary skill in the art would know that additionallayers might be present or that specific layers that are illustratedmight not be present in a device.

As will be discussed, STT-MRAM device 300 has a thin layer of magneticmaterial with perpendicular anisotropy, referred to herein as a thermalstability enhancement layer (TSE) 380. As described herein, the TSElayer is magnetically coupled to the free layer 365 of an MTJ 355through a separation layer 375. A ferromagnetic coupling layer 345 onthe opposite side of fixed magnetic reference layer 340 of MTJ 355provides additional interfaces to achieve high IPMA, which allows thefree layer to maintain an out of plane magnetization, which achieves aperpendicular TMR value above one hundred (100) percent.

To manufacture STT-MRAM device 300, in step 202, a bottom electrode 305is fabricated on a semiconductor wafer (not shown) or other appropriatesubstrate structure. In an embodiment, bottom electrode 305 can comprisesix TaN/CuN multilayers 310, each of which can have a thickness of sixnanometers. These TaN/CuN multilayers 310 are deposited during step 204using magnetron sputtering. Bottom electrode 305 can also comprise TaNlayer 315 fabricated over the TaN/CuN multilayers 310, which in anembodiment can have a thickness of two nm. TaN layer 315 is depositedduring step 206 by magnetron sputtering.

After fabricating bottom electrode 305, perpendicular syntheticantiferromagnet (pSAF) structure 320 is fabricated during step 208. Asseen in FIG. 2, fabrication of pSAF 320 can include several steps, whichwill now be discussed. At step 210, a first magnetic pSAF layer 325 isfabricated. In an embodiment, first magnetic pSAF layer comprises aCo/Ni multilayer having perpendicular anisotropy. First magnetic pSAFlayer 325 is deposited over TaN layer 315 of bottom electrode 305. Firstmagnetic pSAF layer 325 is a magnetic layer having a magnetic directionperpendicular to its plane, as shown in FIG. 3. In an embodiment, firstmagnetic pSAF layer 325 comprises six Co/Ni layers, with each Co/Nilayer having a thickness of 0.8 nanometers.

Fabrication of pSAF 320 further includes step 212, where a non-magneticexchange coupling layer 330 is deposited over first magnetic pSAF layer325. In an embodiment, non-magnetic exchange coupling layer 330 iscomprised of a Co/Ru/Co multilayer comprising a first and second 0.18 nmlayer of Co separated by a 0.85 nm layer of Ru. Next, at step 214, asecond magnetic pSAF layer 335 is deposited over exchange coupling layer330. In an embodiment, second magnetic pSAF layer comprises a Co/Nimultilayer having perpendicular anisotropy. Second magnetic pSAF layer335 is a magnetic layer having a magnetic direction perpendicular to itsplane, as seen in FIG. 3. Second magnetic pSAF layer 335 can comprisefour Co/Ni multilayers, with each Co/Ni multilayer having a thickness of0.8 nanometers. As shown in FIG. 3, the magnetic directions of firstmagnetic pSAF layer 325 and second magnetic pSAF layer 335 are in anantiparallel arrangement with respect to each other due toantiferromagnetic coupling through exchange coupling layer 330. Notethat each or either of the magnetic pSAF layers 325 and 335 in the pSAF320 can be substituted with Co/Pt layers or combination of both Co/Niwith Co/Pt multilayers.

Fabrication of pSAF 320 further can include step 216, in which thereference layer 340 of perpendicular magnetic tunnel junction 355 isfabricated. In an embodiment, the step 216 of fabricating referencelayer 340 comprises step 218, in which a ferromagnetic coupling layer345 is deposited, and step 220, in which a fixed magnetic directionlayer 350 is deposited. In an embodiment, ferromagnetic coupling layer345 can comprise a Co/Ta multilayer, while fixed magnetic directionlayer 350 can be comprised of a layer of CoFeB film and Ta, thecombination of which has perpendicular anisotropy.

The Co layer of Co/Ta ferromagnetic coupling layer 345 can have athickness of 0.21 nm and the Ta layer of Co/Ta ferromagnetic couplinglayer 345 can have a thickness of 0.25 nm. Fixed magnetic directionlayer 350 can comprise layers of CoFeB with a Ta interlayer. In anembodiment, fixed magnetic direction layer 350 comprises a 0.7 nm thicklayer of CoFeB, a 0.25 nm layer of Ta and a 0.8 nm thick layer of CoFeB.Note that Ta interlayer can be substituted by other materials such astungsten (W), hafnium (Hf), etc. Ferromagnetic coupling layer 345 of thereference layer 340 couples fixed magnetic direction layer 350 to secondmagnetic pSAF layer 335 of the perpendicular synthetic antiferromagnetstructure 320, which assists in maintaining the perpendicular magneticdirection of reference layer 340, free layer 365 (to be discussed) andthermal stability enhancement layer 380 (also to be discussed).

At step 222, the remaining layers of perpendicular magnetic tunneljunction 355 are fabricated. As discussed, perpendicular magnetic tunneljunction 355 comprises a reference layer 340 and free layer 365separated by a non-magnetic tunneling barrier layer 360. In anembodiment, non-magnetic tunneling barrier layer 360 is deposited atstep 224. Non-magnetic tunneling barrier layer 360 is comprised of aninsulator material, which can be an approximately one (1) nm thick layerof magnesium oxide (MgO). After depositing non-magnetic tunnelingbarrier layer 360, step 226 is performed, which deposits free layer 365.In these embodiments, free layer 365 can comprise CoFeB layers and Tainterlayer. In an embodiment, a first CoFeB layer has thicknesses of 1.1nm CoFeB, the Ta layer has a thickness of 0.25 nm and the second CoFeBlayer has a thickness of 0.5 nm CoFeB. Note that other materials canreplace the Ta interlayer, examples of which are tungsten (W), hafnium(Hf), etc.

Both free layer 365 and reference layer 340 have perpendicularanisotropy and thus have a magnetic direction that is perpendicular tothe plane of each respective layer. Depending on the logic level storedin the device, magnetic directions of the reference layer and the freelayer will either be parallel or anti-parallel.

At step 230, non-magnetic thermal stability enhancement coupling layer375, the purpose of which will be discussed below, is fabricated overfree layer 365 of perpendicular magnetic tunnel junction. Then, at step232, thermal stability enhancement layer 380 is fabricated overnon-magnetic thermal stability enhancement coupling layer 375. In anembodiment, thermal stability enhancement layer 380 comprises a CoFeBferromagnetic layer having perpendicular anisotropy with a thicknessbetween 0.35 nm and 1.5 nm, while non-magnetic thermal stabilityenhancement coupling layer 375 can comprise a layer of MgO, and can havea thickness that can vary from 0.6-1.2 nm. The choice of MgO thicknessis selected so that different magnetic coupling strengths to free layer365 can be achieved. For MgO thickness of about 0.6 nm to 0.75 nm,magnetic coupling of the thermal stability enhancement layer 380 to thefree layer is strong, and allows optimization of the free layerstability. In one embodiment, non-magnetic thermal stability enhancementcoupling layer 375 is comprised of a 0.7 nm layer of MgO. As thethickness of the MgO of the non-magnetic thermal stability enhancementcoupling layer 375 increases, magnetic coupling will exponentiallydecrease. The thermal stability enhancement layer 380 will become lessand less coupled to the free layer 365 when the thickness of the MgO ofthe non-magnetic thermal stability enhancement coupling layer 375increases and finally decouples when MgO thickness exceeds approximately1.2 nm. A non-magnetic metal or metallic layer can be substituted forthe MgO used in this embodiment for the non-magnetic thermal stabilityenhancement coupling layer 375.

In an embodiment of the device 300 shown in FIG. 3, thermal stabilityenhancement layer 380 comprises a layer of CoFeB having a thickness of0.35 nm deposited over non-magnetic thermal stability coupling layer375. In another embodiment, thermal stability enhancement layer 380comprises a layer of CoFeB having a thickness between 1.3 nm and 1.5 nm.As shown in FIG. 3, thermal stability enhancement layer 380 is magnetic,has perpendicular anisotropy, and has a magnetic direction (illustratedby the arrow) perpendicular to its plane. As will also be discussed, themagnetic direction of thermal stability enhancement layer 380 canswitch, and will generally track the magnetic direction of free layer365.

As discussed, non-magnetic thermal stability coupling layer 375 cancomprise an MgO layer having a thickness of 0.7 nm. The thin CoFeBthermal stability enhancement layer 380 and MgO non-magnetic thermalstability enhancement coupling layer 375 enable improvedrecrystallization of the MgO of non-magnetic tunneling barrier layer 360and the CoFeB of free layer 365 of perpendicular magnetic tunneljunction 355 during the annealing process used to manufacture device300. Enhanced recrystallization of MgO of non-magnetic tunneling barrierlayer 360 and the CoFeB of free layer 365 improves the performance(including thermal stability) of the perpendicular MTJ structure 355.Note that in alternative embodiments, thermal stability enhancementlayer 380 can be construed with materials other than CoFeB, such asalloys of Co, Fe, Ni, or B.

A final step 234 in the process 200 is fabrication of a cappingstructure over the thermal stability enhancement layer 380. Cap 385 cancomprise a 2 nm TaN layer and a layer of Ru having a thickness of 10 nm.

Non-magnetic thermal stability enhancement coupling layer 375 (e.g., anMgO layer) ferromagnetically couples thermal stability enhancement layer380 to free layer 365 and can be used to control degree of stabilizationof free layer 365, thus also allowing indirect tuning of switchingcurrent needed for switching the magnetic direction of free layer 365.The thickness of thermal stability enhancement layer 380 can be adjusted(from 0.1 to 3 nm) to optimize thermal stability and switching currentsfor different device sizes, and the choice of such thickness will beinfluenced by many factors, including the thickness of the MTJ layers.

Thermal stability enhancement layer 380 improves the thermal stabilityof device 300, as will now be discussed with reference to FIGS. 4 and 5.FIG. 4 is a graph of the thin film vibrating sample magnetometer (VSM)major hysteresis loop data for various embodiments of perpendicularmagnetic tunnel junction device. Note that for these tests, the devicessizes for each were the largely the same except for the thickness of thethermal stability enhancement layer 380. The embodiments included adevice having a perpendicular magnetic tunnel junction 355, but with nothermal stability enhancement layer 380. The VSM major hysteresis loopfor this embodiment is labeled as curve 405. A second embodiment is adevice having a perpendicular magnetic tunnel junction 355 and a thermalstability enhancement layer 380 comprising a 1.3 nanometer layer ofCoFeB. The VSM major hysteresis loop for this embodiment is labeled ascurve 410. A third embodiment is a device having a perpendicularmagnetic tunnel junction 355 and a thermal stability enhancement layer380 comprising a 1.5 nanometer layer of CoFeB. The VSM major hysteresisloop for this embodiment is labeled as curve 415.

To obtain this data, a DC field was applied perpendicular to the planeof each. The applied field started at −7000 Oersteds, which thendecreased to 0.00 Oersteds, before rising to +7000 Oersteds, anothervery large magnetic field. The applied field was then decreased steadilyfrom +7000 Oersteds to 0.00 Oersteds, before increasing to −7000Oersteds. Positive and negative signs of the DC applied field indicateperpendicular applied field directions of the field sweep. VSMmeasurements, shown as normalized magnetic moment on the Y axis of thegraph in FIG. 4, were taken with the DC magnetic field applied alongeasy magnetic axis i.e. with magnetic field oriented perpendicular tothe sample plane. The magnetic direction of the perpendicular anisotropyof Co/Ni layer 325 of perpendicular synthetic antiferromagneticstructure 320, Co/Ni layer 335, CoFeB/Ta layer 350 (of reference layer340), free layer 365 and thermal stability enhancement layer 380 areshown for various magnetic field strengths by the arrows in FIG. 4.

As can be seen in FIG. 4, when the applied magnetic field is atapproximately −6500 Oersteds, the magnetic direction of first magneticpSAF layer 325 of perpendicular synthetic antiferromagnetic structure320, second magnetic pSAF layer 335, fixed magnetic direction layer 350(of reference layer 340), free layer 365 and thermal stabilityenhancement layer 380 are all parallel to one another. As the magneticfield decreases to approximately −2500 Oersteds, which is a much largermagnetic field than an MRAM device would experience in real worldapplication, the magnetic direction of second magnetic pSAF layer 335and fixed magnetic direction layer 350 of reference layer 340 switchessuch that it is antiparallel to free layer 365. However, with a largemagnetic field applied, free layer 365 has not switched.

When the applied magnetic field at approximately 0.00 Oersteds movesfrom negative to positive, FIG. 4 shows that the magnetic direction offree layer 365 and thermal stability enhancement layer 380 switch. Themagnetic direction of second magnetic pSAF layer 335 and fixed magneticdirection layer layer 350 of reference layer 340, however, does notswitch. This demonstrates that for each embodiment (e.g., devices withand without a thermal stability enhancement layer 380), the referencelayer 340 does not switch. Thus, FIG. 4 shows that presence of thermalstability enhancement layer 380 does not negatively affect theperformance of reference layer 340 (i.e., reference layer 340 is stilldifficult to switch).

As the applied magnetic field starts increasing, for example, at +5000Oersteds, FIG. 4 shows that magnetic direction of second magnetic pSAFlayer 335 and fixed magnetic direction layer 350 of reference layer 340(and first magnetic pSAF layer 325 of perpendicular syntheticantiferromagnetic structure 320) switch such that the magnetic directionof each of these layers are parallel again. To complete the VSM majorhysteresis loops for each of these embodiments, the applied magneticfield is then reduced to 0.00 Oersteds, where it is then increased, aseach of the curves 405, 410 and 415 in FIG. 4 show. As is seen, theswitching characteristics of each of the first magnetic pSAF layer 325of perpendicular synthetic antiferromagnetic structure 320, secondmagnetic pSAF layer 335, fixed magnetic direction layer layer 350 (ofreference layer 340), free layer 365 and thermal stability enhancementlayer 380 are similar for this portion of the VSM hysteresis loop.

In FIG. 4, curves 405, 410 and 415 show that the magnetic direction offree layer 365 for each embodiment in this example switches atapproximately 0.00 Oersteds. As also seen in FIG. 4, the magnetic moment(the Y axis) for each embodiment is increased for the embodiments havingthermal stability enhancement layer 380 (see curves 410 and 415)compared to the free layer of a perpendicular MTJ device without athermal stability enhancement layer 380 (see curve 405). Thus, bothembodiments constructed with a thermal stability enhancement layer 380over the free layer demonstrated an increase in magnetic volume of theindividual perpendicular MTJ.

Non-magnetic thermal stability enhancement coupling layer 375 betweenthermal stability enhancement layer 380 and free layer 365 provides highinterface perpendicular magnetic anisotropy (IPMA), which acts tomaintain the magnetic direction of both free layer 365 and thermalstability enhancement layer 380 out-of-plane, thus ensuring that themagnetization direction of the thermal stability enhancement layer 380and free layer 365 are perpendicular to their planes.

FIG. 5 compares performance data for devices 300 having a thermalstability enhancement layer 380 against devices without a thermalstability enhancement layer 380. The first row contains median values ofcoercive fields He (in Oersteds) versus device diameter (in nanometers).The second row shows loop shift from zero (0) Oersteds for the magneticfield acting on free layer 365, which is indicative of the magnetostaticcoupling between free layer 365 and perpendicular syntheticantiferromagnet (pSAF) structure having reference layer 340. As with thetests illustrated in FIG. 4, device sizing for each example shown inFIG. 5 were the same except for the presence and thickness of thermalstability enhancement layer 380.

In particular, FIG. 5 compares coercive fields (He) for a device havingno thermal stability enhancement layer 380 (column 1), a device having athermal stability enhancement layer 380 with a thickness of 1.3nanometers (column 2), and a device having a thermal stabilityenhancement layer 380 with a thickness of 1.5 nanometers (column 3). Asis known, coercive field (He) is a good indicator for thermal stability(which is difficult to measure directly). Devices 300 having widths of60 nm, 70 nm, 90 nm and 100 nm were fabricated and tested for eachembodiment. As is easily seen, devices 300 having a 1.3 nanometer thickthermal stability enhancement layer 380 have significantly improvedthermal stability when compared to devices without such a layer.Likewise, devices 300 having a 1.5 nm thick thermal stabilityenhancement layer 380 further improve thermal stability. Indeed, as seenin FIG. 5, coercive fields (Hc) for devices with thermal stabilityenhancement layers 380 increase from approximately 100 Oersteds toapproximately 400 Oersteds. This demonstrates that the thermal stabilityof perpendicular magnetic tunnel junctions is improved when thermalstability enhancement layer 380 is present.

At the same time as coercivity He increases, critical switching currentsare not increased in a manner that might cause performance issues, whichis contrary to what a person having ordinary skill in the art wouldexpect from such a large volume of magnetization. This can be seen inTable 1. In particular, Table 1 shows the critical switching parametersfor the same test devices used to collect the data in FIG. 5:

1.3 nm TSE Layer 1.5 nm TSE Layer Parameter No TSE Layer 380 380 Jc0+/−2.1/2.1 4.3/5.6 5.2/6.5 Vc0+/− 0.36/0.54 0.61/0.94  0.7/0.91 Delta+/−20/22 34/32 30/40 TMR % 100 97 97 Ra Ωμm² 12.3 13 13

In Table 1, Jc0 is the critical switching current density. Vc0 iscritical switching voltage. Delta is the thermal stability factor. TMRis the tunneling magnetoresistance. RA is the resistance area product ofthe tunnel junction. Note also that “+/−” indicates thepositive/negative direction of the perpendicular voltage applied to thetest devices. As the data in Table 1 demonstrates, an MRAM memory cellhaving a perpendicular MTJ and a thermal stability enhancement layer380, where the thickness of thermal stability enhancement layer 380 andfree layer 365 have a combined thickness of 3 nm, achieves both out ofplane magnetization and low critical switching currents.

Note that the embodiment discussed in the context of FIG. 3 has athermal stability enhancement layer 380 constructed of CoFeB, thethermal stability enhancement layer 380 can also be constructed using avariety of other ferromagnetic materials, examples of which are Co, Ni,Fe, and their alloys. In particular CoFeB (Ms ˜1200 emu/cc) or CoFe (Ms˜1500 emu/cc) can be used. By use of or proper alloying of thesematerials, one can achieve desired magnetization values for the thermalstability enhancement layer 380, and thus be able to controlferromagnetic coupling strength and degree of the thermal stability ofthe free layer.

MRAM devices 300 having perpendicular magnetic tunnel junctions andthermal stability enhancement layers 380 as described herein can befabricated using thin film deposition. Layer stacks are deposited bymeans of conventional DC and RF sputtering methods using commerciallyavailable Physical Vapor Deposition (PVD) tools. Layer stacks can beannealed after deposition at 300° C., 1 hour soak time without magneticfield.

In sum, one aspect of the present teachings is forming a magnetic layerthat is magnetically coupled to the free layer of a perpendicularmagnetic tunnel junction. This magnetic layer, described herein as theTSE layer, has a magnetization direction perpendicular to its plane, andseparated from the free layer of an MTJ with a non-magnetic separationlayer.

A person of skill will understand that the above disclosure maps onlyparticular embodiments. It should be further understood that interveninglayers can occur even when one layer is described as having been placedover, is covering, or is on top of another layer. That understandingapplies to the claims. It should further be understood that while theMTJ pillars have been depicted in two-dimensional cross sections, theyare three dimensional objects and the layers discussed may cover thethree dimensional top, all sides, and all surrounding valley portions ofthe MTJ pillars.

It should also be appreciated to one skilled in the art that a pluralityof devices 300 can be manufactured and provided as respective bit cellsof an STT-MRAM device. In other words, each device 300 can beimplemented as a bit cell for a memory array having a plurality of bitcells.

The above description and drawings are only to be consideredillustrative of specific embodiments, which achieve the features andadvantages described herein. Modifications and substitutions to specificprocess conditions can be made. Accordingly, the embodiments in thispatent document are not considered as being limited by the foregoingdescription and drawings.

What is claimed is:
 1. A magnetic device, comprising a bottom electrodein a first plane; a perpendicular synthetic antiferromagnetic structure,the perpendicular synthetic antiferromagnetic structure including amagnetic reference layer in a second plane, the magnetic reference layerhaving a magnetization direction that is perpendicular to the secondplane and having a fixed magnetization direction; a non-magnetic tunnelbarrier layer in a third plane and disposed over the magnetic referencelayer; a free magnetic layer in a fourth plane and disposed over thenon-magnetic tunnel barrier layer, the free magnetic layer having amagnetization vector that is perpendicular to the fourth plane andhaving a magnetization direction that can switch from a firstmagnetization direction to a second magnetization direction, themagnetic reference layer, the non-magnetic tunnel barrier layer and thefree magnetic layer forming a magnetic tunnel junction; a non-magneticthermal stability enhancement coupling layer in a fifth plane anddisposed over the free magnetic layer; a magnetic thermal stabilityenhancement layer in a sixth plane that is physically separated from thefree magnetic layer and coupled to the free magnetic layer by thenon-magnetic thermal stability enhancement coupling layer, the magneticthermal stability enhancement layer having a magnetization directionthat is perpendicular to the sixth plane and having a magnetizationdirection that can switch from the first magnetization direction to thesecond magnetization direction, wherein switching of the magneticthermal stability enhancement layer from the first magnetizationdirection to the second magnetization direction tracks switching in themagnetic free layer; and a cap layer in a seventh plane and disposedover the thermal stability enhancement layer.
 2. The magnetic device ofclaim 1, further comprising a current source that directs electricalcurrent through the cap layer in the seventh plane, the magnetic thermalstability enhancement layer in a sixth plane, the non-magnetic thermalstability enhancement coupling layer in the fifth plane, the freemagnetic layer in the fourth plane, the non-magnetic tunnel barrierlayer in the third plane, the magnetic reference layer in the secondplane, and the bottom electrode in the first plane.
 3. The magneticdevice of claim 1 wherein the magnetic thermal stability enhancementlayer comprises a layer of CoFeB.
 4. The magnetic device of claim 1wherein the magnetic thermal stability enhancement layer comprises afilm of CoFeB having a thickness between 1.3 nanometers and 1.5nanometers.
 5. The magnetic device of claim 4 wherein the free magneticlayer comprises CoFeB with a Ta interlayer.
 6. The magnetic device ofclaim 5 wherein the free magnetic layer has a sum thickness of 1.6nanometers.
 7. The magnetic device of claim 1, wherein the perpendicularsynthetic antiferromagnetic structure further comprises a first magneticpSAF layer and a second magnetic pSAF layer, where the first magneticpSAF layer is over the first electrode and is separated from the secondmagnetic pSAF layer by an exchange coupling layer.
 8. The magneticdevice of claim 7, further comprising a ferromagnetic coupling layer inbetween the second magnetic pSAF layer and the magnetic reference layer.9. The magnetic device of claim 1 wherein the magnetic thermal stabilityenhancement layer is magnetically coupled to the free magnetic layer bythe non-magnetic thermal stability enhancement coupling layer.
 10. Themagnetic device of claim 1 wherein the non-magnetic thermal stabilityenhancement coupling layer comprises a layer MgO.
 11. The magneticdevice of claim 10, wherein the layer of MgO has a thickness between0.6-1.2 nm.
 12. The magnetic device of claim 10, wherein the layer ofMgO has a thickness 0.7 nm.
 13. The magnetic device of claim 1, whereinthe non-magnetic thermal stability enhancement coupling layer provideshigh interface perpendicular magnetic anisotropy between the magneticthermal stability enhancement layer and free magnetic layer such thatthe magnetic direction of the free magnetic layer remains perpendicularto the fourth plane and the magnetic direction of the magnetic thermalstability enhancement layer remains perpendicular to the sixth plane.14. A magnetic device, comprising: a perpendicular magnetic tunneljunction having a magnetic reference layer and a magnetic free layer,the magnetic reference layer and the magnetic free layer separated by anon-magnetic tunneling barrier layer, the magnetic reference layerhaving a fixed magnetic direction that is perpendicular to its plane,the magnetic free layer having a variable magnetic direction that canswitch between a first perpendicular magnetic direction and secondperpendicular magnetic direction, wherein the first perpendicularmagnetic direction and the second perpendicular magnetic direction isperpendicular to the magnetic free layer; a magnetic thermal stabilityenhancement layer disposed over the magnetic free layer of the magnetictunnel junction, the magnetic thermal stability enhancement layercomprising a magnetic material having a variable magnetic direction thatcan switch between the first perpendicular magnetic direction and thesecond perpendicular magnetic direction, wherein switching of themagnetic thermal stability enhancement layer from the firstmagnetization direction to the second magnetization direction tracksswitching in the magnetic free layer; and a non-magnetic thermalstability enhancement coupling layer disposed in between and physicallyseparating the magnetic free layer of the magnetic tunnel junction andthe magnetic thermal stability enhancement layer, the non-magneticthermal stability enhancement coupling layer magnetically coupling thefree magnetic layer and the magnetic thermal stability coupling layer.15. The magnetic device of claim 14, further comprising: an electrode; aperpendicular synthetic antiferromagnetic structure coupled to theelectrode, the perpendicular synthetic antiferromagnetic structureincluding the magnetic reference layer; a cap layer disposed over themagnetic thermal stability enhancement layer.
 16. The magnetic device ofclaim 15, wherein the perpendicular synthetic antiferromagneticstructure further comprises a first magnetic pSAF layer and a secondmagnetic pSAF layer, where the first magnetic pSAF layer is over theelectrode and is separated from the second magnetic pSAF layer by anon-magnetic exchange coupling layer.
 17. The magnetic device of claim14 wherein the magnetic thermal stability enhancement layer comprisesCoFeB.
 18. The magnetic device of claim 14 wherein the magnetic thermalstability enhancement layer comprises a film of CoFeB having a thicknessbetween 1.3 nanometers and 1.5 nanometers.
 19. The magnetic device ofclaim 18 wherein the free magnetic layer comprises CoFeB with a Tainterlayer.
 20. The magnetic device of claim 19 wherein the freemagnetic layer has a sum thickness of 1.6 nanometers.
 21. The magneticdevice of claim 15, further comprising a current source that directselectrical current through the cap layer, the magnetic thermal stabilityenhancement layer, the non-magnetic thermal stability enhancementcoupling layer, the free magnetic layer, the non-magnetic tunnel barrierlayer, the perpendicular synthetic antiferromagnetic structure, and theelectrode.
 22. The magnetic device of claim 14, wherein the non-magneticthermal stability enhancement coupling layer provides high interfaceperpendicular magnetic anisotropy between the magnetic thermal stabilityenhancement layer and free magnetic layer such that the magneticdirection of the free magnetic layer and the magnetic direction of themagnetic thermal stability enhancement layer remain out-of-plane.